Diamond crystal growth apparatus

ABSTRACT

The steady state operating parameters of a low pressure chemical vapor deposition process for making diamond, i.e., nucleation-growth and graphite removal, are applied as controlled sequential steps to favor nucleation and growth.

This application is a division of application Ser. No. 198,966, filedMay 26, 1988 now abandoned.

This invention relates to an improved method for producing diamondcrystals, and more particularly to a chemical vapor deposition forproducing diamond crystals from a hydrogen-hydrocarbon gas mixturebrought into contact with a heated substrate, in which the operatingprocess parameters or conditions are varied in a cylindrical manneralternating between conditions which favor diamond crystal nucleationand growth and conditions which favor gasification of codepositedgraphite.

BACKGROUND OF THE INVENTION

Reference is made to abandoned application Ser. No. 944,729--Anthony etal, filed Dec. 22, 1986, assigned to the same assignee as the presentinvention, which discloses an improved method and apparatus by whichdiamond crystals are caused to nucleate and grow on a preferredsubstrate by means of a heated filament and luminescent gas plasmaactivated hydrogen-hydrocarbon gas mixture coming into contact with thesubstrate. The copending application discloses that ahydrogen-hydrocarbon gas mixture subjected to concurrent activation byan incandescent tungsten wire electrical resistance heater and byelectromagnetic microwave energy becomes a luminescent gas plasma with asignificant atomic hydrogen content. The activated gas mixture isbrought into contact with a heated substrate, and as a consequencethereof, diamond crystals are formed or nucleated on the substrate fromthe gas mixture with subsequent diamond crystal growth. The notedprocess is referred to as a chemical vapor deposition, CVD, process.From the copending application, it is also known that diamonds may beobtained from the described CVD process without the use of microwaveenergy, as shown in U.S. Pat. No. 4,434,188 which described a CVDprocess of causing diamond nucleation and growth from a heated gasmixture in contact with a substrate.

In general, the CVD process involves selection of operating parameterssuch as selection of a precursor gas and diluent gases, the mixtureproportions of the gases, gas temperature and pressure, the substratetemperature and means of gas activation. When these parameters areadjusted to provide diamond nucleation and growth on a substrate, thereis, as an unavoidable consequence of the process, codeposition ofgraphite on the substrate along with the diamond. When the parametersare adjusted to provide for removal of the graphite by gasification orminimal deposition or graphite, diamond nucleation and growthsignificantly decrease. The known CVD processes for producing diamondcrystals can be described as continuous processes which attempt toestablish those conditions of pressure, temperature, and gas mixturewhich provide an acceptable rate of diamond crystal nucleation andgrowth, and only minimal graphite codeposition.

In order to grow diamond crystals at a reasonably high rate in the CVDprocess as described, the operating parameters are usually set as acompromise between deposition of graphite and diamond crystal growth, orbetween diamond growth and graphite removal. Ordinarily, such acompromise may be acceptable where the operating parameters can be setto establish conditions under which graphite codeposition is restrainedand some diamond crystal growth continues, but means are needed toachieve graphite removal in a shorter time period, e.g., providingmaximum short term clean up without overall reduction of high ratediamond growth.

OBJECTS OF THE INVENTION

It is an object of this invention to provide sequential application ofdiamond growth and graphite gasification conditions in a CVD process forproducing diamond crystals.

Another object of this invention is to provide an improved CVD processfor producing diamond crystals under conditions which favor high growthrate followed by optimal removal of codeposited graphite.

It is yet another object of this invention to provide an improved CVDprocess for producing diamond crystals in which those operatingparameters which favor graphite clean up and those parameters whichfavor diamond crystal growth, are applied in a sequential manner withinthe CVD process so that more of the process time can be allocated todiamond crystal growth.

SUMMARY OF THE INVENTION

Diamond crystals are produced in a CVD process by subjecting ahydrogen/hydrocarbon gas mixture to activation by an incandescenttungsten filament wire heater while exposing or contacting a substratesurface with the activated mixture for diamond crystal nucleation andgrowth. The operating conditions of certain condition-controlled stepsor phases of the process such as a high diamond growth rate step and agraphite gasification step are applied in a cyclic or sequential mannerto alternately favor the high growth rate and thereafter gasification.

It will be understood from this disclosure that gasification refers toremoval of co-deposited graphite or carbon from the diamond depositwithout substantial loss of diamond.

According to the present invention, diamond is produced by thermaldecomposition of a hydrocarbon-hydrogen gas mixture at low pressure in achemical vapor deposition apparatus. The overall process which, forconvenience, can be described as a cyclic process comprises a diamondgrowth phase or step which is carried out under conditions which promotenucleation and growth of diamond crystals and a second phase or stepwhich can be described as a purification phase during which co-depositedgraphite is gasified and removed from the diamond material. This secondphase is carried out under conditions which favor removal of graphite.

In general, deposition or growth conditions comprise ahydrocarbon-hydrogen gas mixture in which the amount of hydrocarbon gas,the carbon source, is maintained at a level which is sufficient toprovide maximum growth. During this phase of the cycle, the substrate isheated by a resistance-heated filament to a temperature sufficient tosustain diamond growth. The filament temperature is preferably betweenabout 2000° C. and 2500° C. The substrate temperature is at least, inpart, dependent on the filament temperature and the thermal stability ofthe substrate material. In general, substrate temperatures in the rangeof about 900° C. to about 1100° C. are preferred, particularly for amolybdenum substrate.

The actual amount of hydrocarbon in the gas mixture can be varied toprovide sufficient carbon atoms for conversion to diamond. Suitablehydrocarbons include methane, acetylene, ethane, lower aliphaticalcohols, and the like. For methane, a preferred mixture for the growthphase comprises from about 2 to about 25 volume percent hydrocarbon, thebalance being hydrogen. A particularly preferred mixture comprises about18 volume percent methane. For other hydrocarbons, the amount ofhydrocarbon can be adjusted to provide equivalent atom percent carbon.

While about 2 volume percent hydrocarbon is the upper limit of thecontinuous process presently practiced, the present process is amenableto a hydrocarbon-rich deposition phase gas mixture. The termhydrocarbon-rich is intended to include gas mixture compositions havingsubstantially more than about 2 volume percent hydrocarbon. For example,gas mixtures comprising from about 10 to about 25 volume percent, thebalance hydrogen, and one or more inert gases, are within the term"hydrocarbon-rich".

During the second phase, removal of co-deposited graphite is promoted byexposure to an environment comprising hydrogen, a substantial proportionbeing atomic hydrogen and essentially no hydrocarbon, i.e., by providingan essentially pure hydrogen flow to the filament-substrate region ofthe chemical vapor deposition apparatus.

It will be appreciated that the cyclic process method of this inventioncan be carried on by at least two techniques, which for convenience, arereferred to as physical modulation and temporal modulation.

The physical modulation embodiment involves physical movement of thesubstrate through zones or regions in which the existing conditions aresequentially growth phase or gasification phase. The conditions in eachzone remain constant while the substrate shuttles or passes from one tothe other.

In the temporal modulation embodiment, the substrate is maintained in astationary position and the conditions, particularly the gas mixturecomposition, are varied in order to change the conditions to which thesubstrate is exposed, thus subjecting the substrate to alternatinggrowth phase and gasification phase conditions.

In both embodiments, the relative exposure to growth phase conditionsand gasification phase conditions are measured by time, each beingbalanced so that the gasification phase exposure is sufficient tosubstantially remove the co-deposited graphite from the growth phase.

In the one embodiment, the entire substrate is contained in a singlereaction chamber while conditions favoring high rate growth followed bygraphite gasification are varied or modulated in sequence or time. Inthe other embodiment, the substrate which can be circular in design orcarried on a circular stage rotates through adjacent chambers of thereactor in which constant conditions are set and maintained, onefavoring high growth rate and the other gasification of graphite. Thesize of each chamber can be designed to cooperate with speed of rotationthereby allowing maximum growth time and minimum, though sufficient,gasification time.

It will be appreciated that the objects of this invention are achievedby a process which provides maximum residence time in a high diamondgrowth rate environment, e.g., high hydrocarbon gas concentration, thusavoiding the shortcomings of the prior art continuous process withrespect to the requirement that the rates of graphite deposition andgasification balance; that balance being achieved at the expense of thediamond growth rate, e.g., by using a low-hydrocarbon orhydrocarbon-starved environment.

The embodiments of the invention will be more fully understood whenconsidered in view of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a rotary apparatus which embodiesthe invention and in which the cyclical concept of this invention may bepracticed.

FIG. 2 is a schematic cross-sectional view of the apparatus of FIG. 1taken along the line 2--2 thereof.

FIG. 3 is a schematic top view of the apparatus of FIG. 1.

FIG. 4 is a schematic illustration of a modification of the apparatus ofFIG. 1.

FIG. 5 is a schematic cross-section of the apparatus of FIG. 4, takenalong line 5--5.

FIG. 6 is a schematic plan view of the apparatus of FIG. 5.

FIGS. 7(a) and 7(b) show a schematic representation of time modulatedCVD reaction apparatus with associated schematic representation of theflow rate, reactor pressure, and pumping speeds at the indicated pointsin the process.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An effective apparatus for the practice of the invention is shown inFIG. 1. This apparatus is effective to establish and maintain,simultaneously, the two most important cycle conditions of a CVD processof producing diamond crystals, i.e., diamond growth and graphitegasification conditions. Such conditions can be separately butconcurrently applied to the substrate on which diamonds are beingproduced so that essentially no process time is lost in effectingparameter adjustments required to change from one set of conditions toanother.

Referring now to FIG. 1, a rotary apparatus 10 is disclosed in which thepractice of this invention may be expeditiously carried out. Apparatus10 comprises hollow quartz cylindrical reaction chamber 11 closed off atopposite ends by stainless steel plates 12 and 13. Plate 12 is describedas a gas inlet plate and plate 13 is described as a base plate. Plates12 and 13 are sealed to chamber 11 by a suitable material such as asilicone rubber. Projecting through plate 12 and suspended in electricalinsulating relationship therefrom by insulators 2, are four molybdenumrods, 14, 15, 16 and 17 (16 and 17 not shown) arranged in a rectangularconfiguration with their free ends spaced from plate 13. Each rodincludes a small transverse aperture 18 therethrough adjacent the freeend. Each rod also has the combination of a set screw 19 and a threadedaxial aperture therein at its free end. Suspended from plate 12 is arectangular quartz partition member 20 which is positioned centrally ordiametrically in and, supported by cylinder 11, between a pair ofmolybdenum rods 14 and 15.

Partition member 20 includes a free end 21 which is also spaced fromplate 13 in the same manner as rods 14-17 have their free ends spacedfrom plate 13. The spacing of rods 14-17 and partition 21 from plate 13provides an uninterrupted cylindrical space 22 at one end of chamber 11.A drive shaft 23 projects through base plate 13 into cylindrical space22 and is adapted for rotation in base plate 13 in an appropriatemechanical feed through 24 having minimal gas leakage characteristics.Concentrically mounted on shaft 23 for rotation therewith in cylindricalspace 22, is a circular turntable disc 26 positioned in close proximityto the free ends of rods 14-17 and free end 21 of partition 20. Shaft 23is conveniently rotated by means of a small electrical motor, not shown,e.g., below the base plate. Diametrical partition member 20 engagescylinder 11 along a diameter thereof and effectively divides chamber 11into separate cells 27 and 28. The gas admixture is introduced into cell27 by means of aperture 29 in gas inlet plate 12 and is exhaustedtherefrom by means of aperture 30 in base plate 13. Similarly chamber 28includes a gas inlet aperture 31 in gas inlet plate 12 and a gas outlet32 in base plate 13.

In a general practice of this invention, a predetermined gas or gasmixture is introduced into cells 27 and 28 through gas inlets 29 and 31to flow axially through chamber 11 and exit through gas exits 30 and 32in base plate 13 to provide a slowing gas stream in cells 27 and 28. Theflowing gas is caused to come into contact with incandescent electricalresistance tungsten wire heaters 33 and 34 which are more clearlyillustrated in FIG. 2. Referring now to FIG. 2, electrical resistancetungsten wire filament heaters 33 and 34 are shown as suspended betweenmolybdenum rods 14 and 16 in chamber 28, and 15 and 17, in cell 27.

The gas flow in each cell 27 and 28 comes into contact with heaters 33and 34 and impinge a substrate which is best described with respect toFIG. 1.

In FIG. 1, a substrate in the form of a thin disc 35, preferably ofrefractory metal such as molybdenum, is attached to turntable 26 byscrews 36 to rotate with turntable 26. The arrangement of the notedelectrical resistance heaters 33 and 34 with disc 35 and rods 14-17 arebest described with respect to FIG. 2.

Referring again to FIG. 2 which is view of FIG. 1 taken along the line2--2 thereof, the four rods 14-17 are illustrated as positioned one ateach corner of a rectangle with a heater coil 33 suspended betweenadjacent closer spaced rods 14 and 16, and 15 and 17 by means of thenoted set screws 19 (FIG. 1). Each tungsten wire heater is in the formof an extended fine wire helical coil with straight wire end sectionswhich are inserted in an aperture 18 (FIG. 1) of each of a pair of rodsand retained therein by set screws 19 (FIG. 1) which progresstransversely into apertures 18 to engage the wire end therein.

The apparatus of this invention provides means to separately establishand maintain, in a CVD diamond process, diamond growth, D, conditionsand graphite gasification or clean up, G, conditions, both as constantand steady state processes, while permitting sequential or cyclicalapplication of these conditions to diamond production. Referring againto FIG. 1, by utilizing the same gas pressure in each cell 27 and 28,there is little, if any, flow of gas from one cell to the other underthe free end 21 of partition 20.

In a practice of this invention, optimum diamond nucleation and growthconditions, D, are established in one cell, for example cell 27, andoptimum graphite clean up conditions, G, are established in the othercell 28. The following parameters are relevant for establishing D and Gconditions:

1. Kind of gas or gases utilized.

2. The use of a mixture of gases.

3. Proportions of the gas mixture.

4. Gas flow rate, pressure and temperature.

5. Substrate temperature.

For D conditions, the gas is usually a mixture of high purity hydrogen(above about 99.0%) and hydrocarbon gases, and for G conditions, highpurity hydrogen is the main or sole gas. Substrate temperature is theresult of impingement of hot gases thereon and radiation from the heatercoils both of which are relevant to substrate temperature. The gas incells 27 or 28 is heated by coming into contact with heaters 33 and 34which are at their incandescent temperature, about 2000 degrees C., andpositioned close to substrate 35. In some instances, it may be desirableto use an inert gas such as helium as a diluent for the hydrocarbon gas,i.e., a helium-hydrocarbon mixture in place of a hydrogen-hydrocarbonmixture. Addition of helium gas reduces the amount of atomic hydrogenformed under diamond growth conditions. Substrate material isadvantageously a refractory metal such as molybdenum. Turntable 26 iscaused to rotate, for example, about 2.0 RPM. Under these conditions,substrate 35 is sequentially exposed to each set of conditions D and Gin cells 27 and 28 but only for a time period which, in conjunction withprecise parameter adjustments will optimize the desired crystal growthrate effects, i.e., maximum rate of diamond growth in one chamber andmaximum rate of graphite gasification in the other chamber. Rotation ofturntable 26 represents relative motion between the defined chambers 27and 28 and substrate 35 on turntable 26.

It will be apparent that the described apparatus can be modified so asto carry out the method or process of the invention in devices ofdifferent construction. For example, the apparatus of FIG. 1 may bemodified so that the shaft and turntable are stationary and the otherparts rotate. Alternatively, a substrate in the form of a linear surfacesuch as a conveyor belt may pass under an extended linear array of cellsto provide the cyclical D and G concept of this invention.

A practice of this invention was carried on as follows utilizing theapparatus of FIG. 1. Gas exit apertures 30 and 32 were connected by acommon conduit to a vacuum pump which reduced the pressure in reactionchamber 11 to about 1.0 torr. From an appropriate supply of gases,hydrogen gas was caused to flow into cell 27 at the rate of 36 cm³ /min.and into cell 28 at 44 cm³ /min. and methane was introduced in cell 27at 8.0 cm³ /min., bringing the pressure to about 10 torr. From anappropriate source of electrical power, a current of 26 amperes wascaused to flow in each heater 33 and 34.

The results of two runs are set forth in Table 1, identified as Runs 30and 57. Run 10 was a single chamber continuous run of the type describedin copending application Ser. No. 944,729.

                  TABLE 1                                                         ______________________________________                                        Experimental Conditions                                                       Run            30        57      10                                           ______________________________________                                        Process        Cyclic    Cyclic  Continuous                                   Substrate Rotation                                                                            2         2       0                                           Rate (rpm)                                                                    Heat Treated   No        Yes     No                                           Substrate                                                                     Filaments Supported                                                                          No        Yes     No                                           Their at Midpoints                                                            Filaments Heights                                                                            0.3-1.0     0.3   0.7-1.0                                      (cm)                                                                          Filament Currents                                                                            26        26      26                                           (amps)                                                                        H.sub.2 Flow Rate                                                                            44        44      40                                           (chamber 28)                                                                  CH.sub.4 Flow Rate                                                                            0         0       4                                           (chamber 28)                                                                  H.sub.2 Flow Rate                                                                            36        36      40                                           (chamber 27)                                                                  CH.sub.4 Flow Rate                                                                            8         8       4                                           (chamber 27)                                                                  ______________________________________                                         *flow rates are in standard cm.sup.3 /min                                     filament heights are measured from lowest point of coil                       (ranges reflect height changes during run)                               

In the practice of the herein described CVD diamond process, somecompetition occurs at the substrate for the carbon in the gas mixture.It has been found that a significant amount of carbon diffuses into thesubstrate material and therefore is not available for the diamondnucleation and growth. Carbon diffusion and diamond production processesare considered competing processes. One method for minimizing carbondiffusion involves pretreatment of the substrate, e.g., by heating to anelevated temperature in the range of about 900° C. to about 1200° C. ina hydrocarbon gas environment such as natural gas or methane to saturatethe substrate with carbon prior to its use in the CVD process.

Heater coils 33 and 34 as illustrated in FIGS. 2, 3, and 4, are woundfrom 218 tungsten wire of 0.020 in. D. in a helical coil form with anI.D. of 0.1875 in. and a length of 1.0 in. and straight sections of 0.5in. at each end of the coil which fit into apertures 18 as describedwith respect to FIGS. 1-4. During the deposition process electricalpower input to heaters 33 and 34 raises the temperature causing theheaters to become incandescent. The gas in cells 27 and 28 comes intocontact with and is rapidly heated before impinging on substrate 35. Thetemperature of heaters 33 and 34 is about 2000° C. and substrate 35temperature is above about 800° C. Accordingly, the gas temperaturebetween a heater element and substrate 35 is in the range of from aboveabout 800° C. to a temperature approaching 2000° C. In the cyclicprocess as described, graphite gasification proceeds in one chamber suchas cell 28 in a pure hydrogen atmosphere, and diamond growth occurs inthe other cell 27 in a hydrogen-hydrocarbon gas mixture. Diamond growthoccurs in cell 27 only during the fraction F, of the cycle period thatany point on the substrate 35 spends in the growth cell 27 generallyunder heater 33. In this connection, a distinction is made between anintrinsic rate, I, based on the cumulative residence time under thegrowth filament and an efffective growth rate, E, based on the fullprocess period. The relationship between these growth rates is given bythe equation E=F×I.

                  TABLE 2                                                         ______________________________________                                        Experimental Results                                                          Run                30      57      10                                         ______________________________________                                        Process                Cyclic  Cyclic                                                                              Continuous                               Diameter of Lar-                                                                          D          43      56    80                                       gest Hemispheres                                                              (microns)                                                                     Process Period                                                                            T           6.0     5.4   7.2                                     (hours)                                                                       Fraction of Cycle                                                                         F          1/6     1/6   1                                        Period Allocated                                                              to Diamond Growth                                                             Applied Process                                                                           A          T/3     T/3   T                                        Time                                                                          Intrinsic Growth                                                                          I (=D/FT)q 43.0    62.0  11.1                                     Rate (microns/hr)                                                             Effective Growth                                                                          E (=D/T)    7.2    10.4  11.1                                     Rate (microns/hr)                                                             Based on Full                                                                 Process Period                                                                Effective Growth                                                                          E' (=D/A)  21.5    31.0  11.1                                     Rate (microns/hr)                                                             Based on Applied                                                              Process Time                                                                  ______________________________________                                    

Table 2 above, shows the improvement in the intrinsic growth rate I ofdiamond crystals obtained by the cyclic practice of this invention ascompared to the prior art continuous process. Similarly the effectivegrowth rate E' is much greater for the cyclic practice. The value givenfor the effective growth rate E needs some qualification. As describedabove with respect to the FIG. 1 embodiment, diamond growth takes placein a 60 degree segment of the cyclic or one revolution of substrate 35(F=1/6). Graphite evaporation takes place in a further 60 degreesegment. Also as described above, these segmented conditions result intwo-thirds of each cycle representing unapplied cycle time. Whenunapplied time is considered in E its value becomes comparable with thevalue for the continuous process. In Table 2 the fraction of cycleperiod F allocated to diamond growth is described with respect to FIG. 2as follows.

Referring now to FIG. 2, the subtended angle defined by the length ofthe heaters 33 and 34 is 60 degrees. This indicates that a point orsmall region on the substrate adjacent one end of a heater moves throughan arc of 60 degrees while being directly exposed to the effects of aheater combined with a prescribed gas flow. 60 degrees represents 1/6 ofone revolution of the substrate and accordingly F=1/6. It has bee foundthat diamonds are found on substrate 35 (FIGS. 1 and 2) along an annularband thereon which is related to the shape of heaters 33 and 34. Thewidth of the annulus is usually two to three times the diameter of aheater coil. In the continuous process where the substrate isstationary, diamonds are found in two localized oblong regions, oneunder each filament.

One of the important advantages of the cyclic process is the virtualelimination of transition time between the deposition and gasificationconditions so that no diamond growth time is lost changing conditions orby prolonged graphite clean up. This invention provides separate andisolated optimum constant state conditions for diamond deposition andgraphite gasification in one apparatus with minimal transition time fromone condition to the other. Each cell 27 and 28 represents a flowing gasstream under conditions which enhance diamond growth or graphite removalas specified. The gas streams impinge on, or are deflected by, thesubstrate so that the one stream provides diamond crystal nucleation andgrowth on the substrate, and the other stream removes codepositedgraphite. A modification of the FIG. 1 apparatus which also provides theseparate and isolated conditions is illustrated in FIG. 4.

Referring now to FIG. 4, which is a side elevation view, apparatus 37comprises a hollow cylindrical reaction chamber 38 sealed at both endswith plates 39 and 40. Within hollow chamber 38 and in the same manneras illustrated in FIG. 1, four molybdenum rods 41-44 are suspended fromplate 39 in spaced relationship to each other within reaction chamber38. An electrical resistance heater 45 is suspended between rods 42 and43 in the same manner as heaters 33 and 34 were suspended from similarrods in the FIG. 1 apparatus. Surrounding rods 42 and 43 and heater 45is another but smaller diameter open ended quartz tube 46. Tube 46 ispositioned axially but eccentrically in cylinder 38 with one of its openends coplanar with an open end of cylinder 38, both of which are sealedto plate 39. The other open end quartz tube 46 is spaced within a fewmillimeters from substrate 35 on a turntable 47 which are similar inconstruction and operation to substrate 35 and turntable 26 of FIG. 1. Agas inlet aperture 48 in plate 39 communicates with tube 46 while othergas inlet apertures 49 communicate with the interior volume of cylinder38 not occupied by tube 46. Accordingly, tube 46 with its own heater 45,and its own gas inlet aperture 48, defines a separate and isolated cell51 within cylinder 38 in which deposition (D) or gasification (G)conditions are established. Also a D or G set of conditions may beestablished in the surrounding cell or volume 52 of cylinder 38 notoccupied by tube 46. This relationship is more clearly shown in FIG. 5which is a view of FIG. 4 taken along the line 5--5 thereof.

Referring now to FIG. 5, apparatus 37 includes the noted hollow quartzcylinder 38 with the smaller diameter quartz tube 46 positioned axiallybut eccentrically therein in surrounding relationship to a pair of rods42 and 43 which suspend a heater coil 45 therebetween. As previouslydescribed, rods 41, 42, 43 and 44 are arranged along an arcuate pathspaced within the parallel to quartz cylinder 38. As illustrated, thearc of heater 45 only occupies a part of the projected circumference ofwhich it is a part. The remaining rods 41 and 44 also support an arcuateheater coil 53 which, together with heater coil 45 defines a circularlength of heater coil, the circumference of which is concentric with andspaced within cylinder 38. The molybdenum rods which support each heater45 and 53 are connected to a source of electrical power (not shown)through separate control means so that electrical power to the heatersmay be separately controlled. As illustrated in FIG. 5, the subtendedangle of tube 46 is 60 degrees so that a point on the substrate 35 underheater 45 moving under tube 46 spends one-sixth of the time required forturntable 35 to make one revolution. High efficiency graphite removal orclean up conditions may be established in tube 46 while in the remainingvolume, deposition conditions are established and maintained. Underthese circumstances, F is about 5/6, as compared to 1/6 for theapparatus of FIGS. 1 and 2.

In FIG. 2, note that only 60 degrees, 1/6 of a revolution of substrate35, is allocated to diamond growth in chamber 28, and only 1/6 or 60degrees of rotation of substrate 35 is allocated to graphitegasification conditions (G) in chamber 27, i.e., a point on substrate 35adjacent heater 34 moves through the subtended angle of 60 degrees.Accordingly, from one full cycle (360 degrees of revolution of substrate35, only 1/3 revolution) is actually applied to D or G and the remaining2/3 of the cycle is unapplied cycle time.

The embodiment of FIGS. 4 and 5 increases the applied cycle time andsubstantially eliminates unapplied time. For example, referring to FIG.5, G conditions are established in tube 46 representing 60 degrees or1/6 of a revolution of substrate 35. However, D conditions areestablished in space 52 representing the balance or 5/6 of a revolutionof substrate 35, and accordingly almost a full cycle is utilized asapplied time, about 1/6 in tube 46 under heater 45 and about 5/6 inspace 52 under heater 53. Tube 46 may be utilized for concentratedgraphite removal and space 52 for diamond growth.

In the apparatus as described in FIGS. 1-6, the substrate is cyclicallyand alternately, and serially and sequentially exposed to two steadystate conditions D and G each of which may have its parameters adjustedfor optimum constant condition results for the time period the substratespends exposed to the conditions in each chamber. Both the D and the Gprocesses are low pressure processes so that, with constant and equalpressure within tube 46 and within cylinder 38 (FIGS. 3 and 4), gasleakage between tube 46 and cylinder 38 is minimized. The cyclic CVDprocess is more adaptable to higher concentration of hydrocarbon thanthe constant or steady state process. For example, Table 1 shows thatfor the same total flow rate, the methane flow rate in the cyclicprocess was 8 cm³ /min. compared to 4 cm³ /min. for the continuousprocess. This invention provides a CVD diamond process apparatusutilizing a method which applies steady state diamond growth (D)conditions, and graphite clean up (G) conditions cyclically oralternatively to a diamond crystal during its growth process.

In the apparatus as described in FIG. 1, the ends of each cell 27 and 28represented by the substrate 35 may be described as coplanar in that theplanar substrate surface 35 serves as an assumed end surface for eachcell to be impinged by the gas stream emanating from each cell 27 and28.

In the FIG. 4 apparatus, in the same manner as described above for FIG.1, substrate 35 serves as a co-planar base to both cells 51 and 52 to beimpinged by the separate gas streams in each cell. In each apparatus ofFIGS. 1-6, substrate 35 is a planar surface positioned transversely tothe open ends of the gas cells. Substrate 35 also moves in a transversedirection so that a surface region thereon is exposed to the gas flow ofeach cell. The procedure is continuously repetitive without any timelost in stopping or any requirement for reversal. The process occurringin each chamber continues as a constant process but the process isapplied cyclically to the carousel-like substrate as each previouslyimpinged surface region or point on the substrate moves into position tobe exposed to the appropriate gas stream.

FIG. 7 shows a chemical vapor deposition reactor 10 having gas inletconduits 12 and 14, fitted with automatic open-closed valves 16 and 18.On the vacuum side of the reactor, conduit 20 is provided with avariable flow valve 22.

For the purposes of description, gas mixture A can be a hydrogen-methanemixture and mixture B can be high purity hydrogen.

Conditions within the system at various points are illustrated by thetime-function data for a complete cycle of the process. For example,when gas mixture A flows through open valve 16, valve 18 is closed. StepA for purposes of description therefore corresponds to a depositionphase. At the end of the deposition phase, valve 16 is closed, valve 22is opened completely to purge the reactor. When reactor pressure isreduced to a predetermined level, valve 22 closes and valve 18 opens,refilling the reactor with hydrogen for completion of the cycle by thegasification phase (Step B). While the time factor for flow of gasmixtures A and B are shown as being equal, it will be appreciated thatthe time factor can be adjusted to provide maximum efficiency for thedeposition and gasification phases.

Optional connection to other similar reactors is within the scope of thedescription.

The method and apparatus of this invention are advantageously utilizedin a modified diamond CVD process which is specifically directed to therapid growth of individual diamond crystals. In the practice of thisinvention as described, as well as in the practice of the invention inthe noted copending application, diamond crystal nucleation is evidencedby the rapid appearance of a profuse number of diamond crystalnucleation sites. Observation of diamond crystal nucleation and growthon a substrate suggests two mechanisms for crystal growth. In the firstmechanism, carbon atoms from the impinging gas stream contributedirectly to crystal growth at these sites. In another mechanism, othercarbon atoms in great numbers impinge on intermediate areas of thesubstrate and migrate along the substrate surface to the nucleationsites to contribute to diamond crystal growth. When there are a greatnumber of nucleation sites, there is competition among adjacent sitesfor these carbon atoms and some sites may receive carbon atoms at theexpense of an adjacent site, i.e., carbon atoms which otherwise wouldhave migrated to an adjacent site. Accordingly, a few number of sitesmay be beneficial as each site may receive a full supply of carbon atomswithout excess loss to adjacent competing sites.

Attempts have been made to limit or predetermine the number of diamondcrystal nucleation sites by roughening the substrate surface in apredetermined manner, for example, by having a prescribed geometricarray of spaced peaks and valleys to encourage nucleation on the peaks,for example. Also, it is known that certain substrate metals are moreconducive to diamond crystal nucleation than others. Refractory metals,molybdenum, for example, are more conducive than other metals such assteel, for example. Furthermore, the operating parameters of the CVDprocess may be adjusted to suppress or limit profuse nucleation.Generally, enrichment of the hydrogen-hydrocarbon gas mixture withmethane gas leads to increased nucleation. The method and apparatus ofthe present invention may be advantageously applied to limit nucleationand therefore concentrate the CVD process on fewer nucleation sites formore favorable crystal growth because the substrate is continuouslyalternately exposed to optimum diamond growth and graphite evaporationconditions for concentration of these optimum conditions on crystalgrowth and graphite evaporation at each site. Fewer nucleation sitesprovides each site with a more generous share of one of the growthmechanisms as described.

In the cyclic process for producing diamond by chemical vapordeposition, the composition of the gas mixture is varied to produce atwo-step cycle. In the deposition step, the mixture is rich inhydrocarbon in the sense that the partial pressure of the hydrocarbonfeedstock in the gas mixture is significantly greater than in the priorart continuous process. For the gasification step, the gas is purehydrogen, which has been found to enhance removal of graphite depositedby the hydrocarbon-rich first step. It has been shown that the increasein the instantaneous growth rate obtained by using the hydrocarbon-richmixture more than compensates for the extra process time needed toremove the graphite by gasification, so that the effective growth rateexceeds the rate for the continuous process.

As described, the cyclic process can be achieved by two embodiments ofthe process. The first embodiment involves temporal variation of the gascomposition. In this embodiment the substrate remains generallystationary while the gas composition is varied in phase over the entiresubstrate. A second embodiment, known as spatial variation involvesmovement of the substrate from one reaction chamber to another, eachwith different gas composition so that as the substrate rotates throughthe chambers, it is alternately exposed to different gas compositions.

The so-called temporal variant of the cyclic process is in many respectsmore convenient for large scale reactors, particularly when the gascomposition is varied on a time scale which is shorter than the gasresidence time in the reactor. FIG. 7 is a schematic representation of asingle chamber reactor which can be varied with time and of the purgerefill cycle. The reactor is fitted with electronically activated flowcontrollers and high vacuum valves. The gas composition is varied byadding to each cycle two purge steps during which the high vacuum valveis opened relatively wide. The object is to briefly depress the pressurefollowing a growth step, so that when the reactor is back filled withthe hydrocarbon-free gas mixture called for in the gasification step,there remains only a very small partial pressure of thehydrocarbon-containing mixture which was used during the depositionportion of the cycle. That partial pressure is essentially equal to thepressure prevailing at the end of the preceding purge step.

While this invention has been described with regard to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the invention as set forth in the following claims.

What I claim is:
 1. An apparatus for the manufacture of diamond by acyclical CVD process comprising in combination:(a) means to provide afirst flowing stream of one or more gases, one of which is a gascontaining carbon; (b) means to provide a second flowing stream of a gasisolated from said first stream and consisting essentially of hydrogen;(c) a substrate surface positioned to deflect and be impinged by saidgas streams; and (d) means to expose a region of said substratesequentially to each said stream on a continuing basis for diamondnucleation and growth on said substrate as well as graphite gasificationfrom said substrate, wherein said means to expose a region of saidsubstrate comprises drive means to rotate said substrate so that saidsubstrate is sequentially exposed to each of said gas streams.
 2. Theinvention as recited in claim 1 wherein said substrate surface is arefractory metal.
 3. The invention as recited in claim 1 wherein saidfirst flowing stream comprises a mixture of high purity hydrogen andmethane.
 4. The invention as recited in claim 1 wherein the temperatureof said first and second flowing stream is increased by contact thereofwith an incandescent tungsten wire electrical resistance heater adjacentsaid substrate.
 5. The invention as recited in claim 1 wherein saidsubstrate is planar.
 6. The invention as recited in claim 5 wherein saidsubstrate is circular.
 7. An apparatus for the practice of a cyclicalCVD process of producing diamond crystals comprising in combination:(a)a hollow cylinder, (b) a gas inlet plate closing one end of saidcylinder, (c) a base plate closing the other end of said cylinder, (d) adrive shaft positioned concentrically in said base plate to project intosaid cylinder for rotation therein, (e) a circular disc substratemounted concentrically on said shaft for rotation in said cylinder, (f)a diametrical partition member positioned diametrically in said cylinderto extend from said gas inlet plate with a free end closely spaced fromsaid circular disc substrate, (g) said diametrical partition dividingsaid cylinder into a pair of side by side open ended cells and definingcoplanar open ends for said cells, (h) an electrical resistance heaterin each said cells and positioned close to but spaced from saidsubstrate, (i) gas inlet means in said gas inlet plate to introduce gasseparately into said cells, (j) gas exit means in said base plate toprovide an egress of gas from said cells and a flow of gas therefrom ina direction from said gas inlet plate towards said base plate to impingesaid substrate and egress from said base plate, and (k) means to rotatesaid drive shaft and said substrate in said cylinder so that each pointon the substrate is impinged by the flow of gas in said cells seriallyand sequentially.
 8. A cyclical apparatus for the practice of a diamondCVD process therein comprising in combination:(a) a hollow open endedcylindrical reaction chamber having a plate closure at one end and anopposite gas inlet plate at the other end, (b) a drive shaft projectingconcentrically through said plate closure and adapted to rotate thereinwith a free end of said shaft in said chamber, (c) a turntable mountedon the said free end of said shaft to rotate concentrically in saidchamber adjacent the closure plate thereof, (d) a planar diametricalpartition member in said chamber extending from the said gas inlet plateand projecting close to said turntable to diametrically divide saidchamber into a pair of side by side cells which are closed at one end bysaid gas inlet plate, (e) said gas inlet plate having gas inletapertures therein to provide separate gas inlet means for each of saidside by side cells, (f) said plate closure having gas exit aperturestherein to provide a flow of gas from each of said cells to impinge saidturntable, (g) heating means in each said side by side cells to increasethe temperature of the gas therein, (h) and drive means to rotate saidturntable to expose each point on the surface of said turntablealternately and sequentially to the flow of heated gas from each of saidside by side cells.
 9. The invention as recited in claim 8 wherein saidheating means comprise a tungsten wire electrical resistance heater. 10.The invention as recited in claim 8 wherein a refractory metal surfaceis attached to said turntable to be exposed to the flow of gas from saidchambers to have diamond crystals nucleate on and grow on saidrefractory metal surface.